1. Field of the Invention
The invention relates to a method and a system for regulating the instantaneous electromagnetic torque of a polyphase rotating electrical machine and an information storage medium for implementing this method.
2. Description of the Prior Art
To be more precise, the invention relates to a regulation method in which the rotating electrical machine is equipped with stator and/or rotor windings supplied by a voltage and a current generated by an inverter formed of switches whose switching is controllable, said method comprising:                a step of commanding the switching of the switches of the inverter using the fundamental frequency of the voltage generated by the inverter and a form of synchronous pulse width modulation configured as a function of an input set point, and        a step of calculating the input set point from an instantaneous torque set point at each regulation time.        
In the remainder of the description, the terms “motor” and “torque” respectively denote a polyphase rotating electrical machine and the electromagnetic torque of this kind of machine. Moreover, the terms “fundamental” and “harmonic” respectively denote the sinusoidal component with the lowest frequency and the harmonics of second or higher order of a signal.
Thus the term harmonic as used herein refers only to harmonics of the second or higher order.
Methods of regulating the instantaneous torque of a motor are used in many fields, for example to control rolling mill drive motors.
There are many methods of using an inverter to chop a direct current voltage to create a polyphase alternating current voltage. Of such methods, those using pulse width modulation (PWM) are the most widely used. Pulse width modulation is divided into two families, respectively denoted “asynchronous” and “synchronous”.
Asynchronous pulse width modulation determines the switching times of the switches of the inverter from the intersection of a high-frequency carrier that is asynchronous with respect to the frequency of the fundamental of the voltage generated by the inverter and a modulating sinusoid at the required stator frequency.
Synchronous pulse width modulation, on the other hand, is synchronized to the fundamental frequency of the voltage generated by the inverter, and is defined by angles defining the times of occurrence and the number of switching operations during each fundamental period of the generated voltage. Typically, since each form of synchronous pulse width modulation corresponds to a particular voltage generated by the inverter, the control unit is adapted to employ several forms of synchronous pulse width modulation. In the present context, each form of synchronous pulse width modulation is defined by the particular value of the angles used to construct it. Thus in the remainder of this description two forms of synchronous pulse width modulation having different angle values each correspond to a different form of pulse width modulation.
During the regulation process, the control unit selects the form of pulse width modulation to use as a function of the amplitude of the voltage that the inverter must generate, for example.
For each form of modulation, the value of the angles used is generally precalculated to eliminate certain predefined harmonics, which is one advantage of synchronous pulse width modulation.
In the case of methods of regulating the instantaneous torque of a motor, the form of modulation is selected as a function in particular of the instantaneous torque set point. It has been found that control methods of this kind can achieve the advantages associated with the use of synchronous pulse width modulation only partially, if at all. This is because, although each form of modulation is constructed to eliminate certain harmonics, this occurs only partially, if at all.
The invention therefore aims to remedy this problem by proposing a method of regulating the instantaneous torque of a motor that uses synchronous pulse width modulation control and retains the advantages associated with using asynchronous pulse width modulation.